The objective of the research is to analyse and more precisely describe various phenomenal that occur in the transmission of viruses by arthropod vectors. The effect of an infection of the aphid vector, Hyperomyzus lactucae, with sowthistle yellow vein virus on DNA synthesis will be investigated. Two methods to be used are: 1) measuring dihydrofolate reductase enzyme activity in infected and noninfected aphids, and 2) seeing if Actinomycin D will affect viral replication in injected aphids. The characterization of the vector-virus relationships of circulative viruses will continue. Transmission of sugar beet yellow-net and potato leafroll viruses by the green peach aphid, Myzus persicae, will be studied. In particular, an attempt will be made to find out if declining inoculativity can be restored by another feeding period on a virus source.